3,452 research outputs found
Magnetic fields in primordial accretion disks
Magnetic fields are considered as a vital ingredient of contemporary star
formation, and may have been important during the formation of the first stars
in the presence of an efficient amplification mechanism. Initial seed fields
are provided via plasma fluctuations, and are subsequently amplified by the
small-scale dynamo, leading to a strong tangled magnetic field. Here we explore
how the magnetic field provided by the small-scale dynamo is further amplified
via the dynamo in a protostellar disk and assess its
implications. For this purpose, we consider two characteristic cases, a typical
Pop.~III star with ~M and an accretion rate of
~M~yr, and a supermassive star with ~M
and an accretion rate of ~M~yr. For the ~M
Pop.~III star, we find that coherent magnetic fields can be produced on scales
of at least ~AU, which are sufficient to drive a jet with a luminosity of
~L and a mass outflow rate of ~M~yr. For
the supermassive star, the dynamical timescales in its environment are even
shorter, implying smaller orbital timescales and an efficient magnetization out
to at least ~AU. The jet luminosity corresponds to
~L, and a mass outflow rate of
~M~yr. We expect that the feedback from the
supermassive star can have a relevant impact on its host galaxy.Comment: Accepted for publication in Astronomy & Astrophysics, comments are
still welcom
The formation of supermassive black holes in rapidly rotating disks
Massive primordial halos exposed to moderate UV backgrounds are the potential
birthplaces of supermassive black holes. In such a halo, an initially
isothermal collapse will occur, leading to high accretion rates of
~M~yr. During the collapse, the gas in the interior
will turn into a molecular state, and form an accretion disk due to the
conservation of angular momentum. We consider here the structure of such an
accretion disk and the role of viscous heating in the presence of high
accretion rates for a central star of , and ~M. Our
results show that the temperature in the disk increases considerably due to
viscous heating, leading to a transition from the molecular to the atomic
cooling phase. We found that the atomic cooling regime may extend out to
several ~AU for a ~M central star and provides substantial
support to stabilize the disk. It therefore favors the formation of a massive
central object. The comparison of clump migration and contraction time scales
shows that stellar feedback from these clumps may occur during the later stages
of the evolution. Overall, viscous heating provides an important pathway to
obtain an atomic gas phase within the center of the halo, and helps in the
formation of very massive objects. The latter may collapse to form a massive
black hole of about ~M.Comment: Accepted for publication in Astronomy & Astrophysics, comments are
still welcom
Northern Hemisphere interdecadal variability: A coupled air-sea mode
A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific.
The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal.
The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration
Witnessing the birth of a supermassive protostar
The detection of quasars reveals the existence of supermassive
black holes of a few . One of the potential pathways to
explain their formation in the infant universe is the so-called direct collapse
model which provides massive seeds of . An isothermal
direct collapse mandates that halos should be of a primordial composition and
the formation of molecular hydrogen remains suppressed in the presence of a
strong Lyman Werner flux. In this study, we perform high resolution
cosmological simulations for two massive primordial halos employing a detailed
chemical model which includes cooling as well as realistic opacities
for both the bound-free emission and the Rayleigh scattering of
hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high
densities of and to scales of about AU.
Our results show that the gas cools down to 5000 K in the presence
of cooling, and induces fragmentation at scales of about 8000 AU in
one of the two simulated halos, which may lead to the formation of a binary. In
addition, fragmentation also occurs on the AU scale in one of the halos but the
clumps are expected to merge on short time scales. Our results confirm that
cooling does not prevent the formation of a supermassive star and the
trapping of cooling radiation stabilises the collapse on small scales.Comment: Accpeted version, to appear in MNRAS, comments are still welcome and
high resolution version is available at
http://www2.iap.fr/users/latif/DCBH.pd
Origins of the midlatitude Pacific decadal variability
Analysis of multiple climate simulations shows much of the midlatitude Pacific decadal variability to be composed of two simultaneously occurring elements: One is a stochastically driven, passive ocean response to the atmosphere while the other is oscillatory and represents a coupled mode of the ocean‐atmosphere system. ENSO processes are not required to explain the origins of the decadal variability. The stochastic variability is driven by random variations in wind stress and heat flux associated with internal atmospheric variability but amplified by a factor of 2 by interactions with the ocean. We also found a coupled mode of the ocean‐atmosphere system, characterized by a significant power spectral peak near 1 cycle/20 years in the region of the midlatitude North Pacific and Kuroshio Extension. Ocean dynamics appear to play a critical role in this coupled air/sea mode
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